The present invention relates to wireless communications systems and methods, and more particularly, to mobility management systems and methods for wireless communications.
An aim of hybrid terrestrial-satellite networks may be for the satellite network to provide a ubiquitous coverage overlay for the terrestrial network. However, this may give rise to a number of mobility management challenges that may not be optimally handled by the current state of the art.
FIG. 1 shows a satellite overlay for a terrestrial network. Three types of regions are identified. A region A is the planned coverage area of the cellular network; typically this will comprise a multiplicity of cell sites and location areas (LAs), served by multiple mobile switching centers (MSCs). “Location area” is a term used in the cellular literature to indicate a collection of cell sites over which paging is performed; an MSC may have several LA's under its jurisdiction. A mobile station (MS) can move about (e.g., change broadcast control channel (BCCH)) within an LA without re-registration, however it typically must re-register when it crosses an LA boundary. This re-registration is called a Location Area Update.
A region C is the planned coverage area of the satellite network, comprising a multiplicity of spotbeams, served by one or more gateways, which act as the satellite equivalent of the MSC. A region B includes holes inside the cellular coverage region A that are also covered by the satellite network. The holes exist mainly in suburban areas and in corridors between urban areas where the population density may not be sufficient for the cellular operator to prioritize the deployment of more base station towers. In dense urban areas, where a substantial amount of traffic is generated, such holes are less common. Note that some holes may be covered by neither network if the satellite view is under heavy blockage.
If the satellite and terrestrial networks are under different administrations, they will typically view each other as different Public Land Mobile Networks (PLMNs) offering “roamable service”. If the networks are under the same administration, then the handover typically is performed as between different MSCs, which is known in the present art. Roamable service means that the services have commercial agreements and technical interfaces set up to allow roaming from one network to another. The fact that these networks involve different frequency bands generally is not a technical impediment to roaming, as such roaming already takes place, for example, in GSM between European 900 MHz and North American 1900 MHz bands.
Essentially, once an MS, e.g., a handset, is unable to locate a single forward control-channel carrier (e.g. BCCH carrier in GSM) in the band that it last used, it will start scanning carrier frequencies in alternate bands, like the satellite band. Which bands the MS scans, and in what sequence, is typically determined by a native login application in the MS. Once the MS finds a suitable forward control-channel carrier, it will camp on that carrier and control channel. Camping may include synchronization to the carrier and control channel, and registering to the network for service. Pages for incoming calls to the MS will be sent on the camped-on forward control channel.
The roaming approach described above may be adequate when the MS moves from the cellular coverage region A to the satellite coverage regions C in the idle mode, as it is a quasi-permanent change in the serving network for the MS and very rapid network change may not be necessary. However, there may be at least two cases when a very rapid change in the serving network, and/or a change in network without explicit registration may be desirable.
FIG. 1 shows a highway passing through a number of holes B in the cellular coverage area A. A large number of MS's might be passing through the holes B in idle mode and with no need for satellite communications. However, as each of the MS's may be programmed to roam to the satellite network if the terrestrial service is unavailable, each MS, as it passes through the hole, may attempt to perform an inter-PLMN roam, involving a Location Area Update and Registration. This could create a huge and unnecessary burden on the satellite resources of power and bandwidth, which may be unnecessary, as most of the MS's may not need to use the satellite network.
If an MS were moving from the cellular coverage region A to a hole B or the satellite coverage region C (i.e., any region where there is no cellular coverage) while it was engaged in a call, the call, typically, would be dropped. Call dropping is generally considered a very negative user experience in cellular services and, if the user has been told that there is a satellite coverage-overlay, he may expect the same seamless service as in cellular. After being dropped, the MS (if it stayed sufficiently long in the new region) would typically roam to the satellite network and camp on to a satellite forward control channel. As the mobile re-entered the cellular network it would typically roam back again to the cellular network, which again could involve a significant period of service unavailability for the user. This means that when the MS is in the satellite network, it may periodically search for cellular control-channel carriers and roam to these, if available. This searching may be performed by a variety of means described in the present art, including adjacent cell monitoring as in GSM.